Unable to copy into Compose

Compose has changed on my systems as of yesterday, it will not let me copy (CtlV) text into the body of the email, but it will accept CtlV on To: and Subject:

Reload and reboot makes no difference.

This does not seem like a bug, a setting or a new 'enhancement'...

- It has lasted longer than 24 hours so seems an unlikely bug to have lasted this long
- A setting to prevent copying seems ludicrous and have not changed settings in years
- Deliberately preventing copy into the body of an email is so ridiculous surely no one at Yahoo is this clueless>